fix: add pytidb[models] deps to memory example

[yahonda]

This commit adds litellm to memory example to address ModuleNotFoundError: No module named 'litellm' error.

Steps to reproduce

Follow the steps described at https://github.com/pingcap/pytidb/tree/main/examples/memory#how-to-run

Expected behavior

python main.py should run without errors.

Actual behavior without this fix

It raises the "ModuleNotFoundError: No module named 'litellm'"

(.venv) ~/src/github.com/pingcap/pytidb/examples/memory % python main.py

Traceback (most recent call last):
  File "/Users/yahonda@pingcap.com/src/github.com/pingcap/pytidb/examples/memory/main.py", line 22, in <module>
    embedding_fn = EmbeddingFunction(
        model_name="text-embedding-3-small", api_key=os.getenv("OPENAI_API_KEY")
    )
  File "/Users/yahonda@pingcap.com/src/github.com/pingcap/pytidb/examples/memory/.venv/lib/python3.13/site-packages/pytidb/embeddings/litellm.py", line 69, in __init__
    self.dimensions = len(self.get_query_embedding("test"))
                          ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/Users/yahonda@pingcap.com/src/github.com/pingcap/pytidb/examples/memory/.venv/lib/python3.13/site-packages/pytidb/embeddings/litellm.py", line 72, in get_query_embedding
    embeddings = get_embeddings(
        api_key=self.api_key,
    ...<4 lines>...
        input=[query],
    )
  File "/Users/yahonda@pingcap.com/src/github.com/pingcap/pytidb/examples/memory/.venv/lib/python3.13/site-packages/pytidb/embeddings/litellm.py", line 29, in get_embeddings
    from litellm import embedding
ModuleNotFoundError: No module named 'litellm'
(.venv) ~/src/github.com/pingcap/pytidb/examples/memory %

Actual behavior with this fix

It works as expected that Chat with AI shows the prompt below.

(.venv) ~/src/github.com/pingcap/pytidb/examples/memory % python main.py

Chat with AI (type 'exit' to quit)
You:

[Mini256]

Using pytidb[models] instead.